Aphakia is the absence of the crystalline lens of the eye. The cause of that disability is normally the result of an operation to remove cataracts from the lens of the eye. The crystalline lens of the eye provides protection to the retina against short wave (ultraviolet) radiation since it acts to strongly absorb radiations having wavelengths shorter than about 420 nm. Hence, a lens exhibiting near-zero transmittance of wavelengths shorter than about 440 nm would be of great benefit to aphakics.
Retinitis has been generally defined as inflamation of the retina. Retinitis may be due to infectious agents (customarily bacteria, less commonly fungi, rarely viruses), mechanical injury of the eyeball (contusion), intense light (photoretinitis), or to chronic progressive damage to the retina (degenerative retinopathy).
Photoretinitis may follow direct observation of intense light sources such as the sun, electrical arcs, welding operations, or even reflected sunlight (snow blindness). The resulting blind spot is central and is usually temporary, but it may be permanent if the exposure is severe enough. It has been discovered that photoretinitis resulting from prolonged exposure to the sun's rays is not caused by thermal injury but, rather, is primarily due to radiations of short wavelengths in the sun's spectrum. Consequently, in like manner to aphakia, a spectacle lens capable of cutting off radiations in the ultraviolet and near-ultraviolet regions of the spectrum would be beneficial. Therefore, a lens exhibiting near-zero transmittance of wavelengths shorter than about 440 nm would also be highly desirable for persons subject to photoretinitis.
Degenerative retinopathy is frequently associated with systemic diseases of the blood vessel system; e.g., arteriosclerosis, hypertensive cardiovascular and kidney diseases, diabetes, leukemia, and anemia. However, retinitis pigmentosa is one type of degenerative retinal infection which has been defined as a hereditary affection inherited as a sex-linked recessive characterized by slowly progressing atrophy of the retinal nerve layers and clumping of retinal pigment followed by attenuation of the retinal arterioles and waxy atrophy of the optic discs. In general, the disease results in the gradual impairment of peripheral and night vision. The rate of progression varies widely in individual patients such that some persons retain useful vision up to 40-50 years of age. Where the disease has progressed rapidly, the loss of peripheral and night vision has caused grave psychological and social problems during childhood and adolescence. There is no known effective treatment for the disease, the growth of pigment cells continuing into and over the inner surface of the retina resulting in ultimate scarring and blindness.
The eye contains two different kinds of photoreceptors, viz., cones and rods. The cones comprise the principal receptors in daylight vision (photopic vision) and the rods constitute the principal receptors in night vision (scotopic vision).
The cones are located generally in the central portion of the retina and permit the recognition of fine detail, presumably because they function largely independently of one another. The cones also permit color vision, i.e., they allow hues and saturation to be distinguished. In the presence of bright light, the eye is most sensitive to radiations at about 555 nm.
In general, the rods are located in the peripheral portions of the retina, there being few if any rods found in the central retina. The rods do not permit the recognition of colors, only shades of gray. Their peak sensitivity is at a wavelength of about 510 nm.
FIG. 1 depicts the relative wavelength response for photopic and scotopic vision.
Published medical research, e.g., "Photic Damage in Retinitis Pigmentosa and a Suggestion for a Protective Device", Werner Adrian and Ingeborg Schmidt, Journal American Optometric Association, 46, 380-386, April, 1975, and "Protective Spectacles for Retinitis Pigmentosa Patients", Ronald Everson and Ingebord Schmidt, Journal American Optometric Association, 47, 738-744, June 1976, has indicated that a spectacle lens having reduced photopic (luminous) and strongly reduced scotopic transmittance might provide relief to persons suffering from retinitis pigmentosa.
It has been generally accepted that retinal degeneration can be decelerated by preventing both receptors of the retina (cones and rods) from being subjected to bright illumination and great changes in light levels. The continuously decreasing quality of night vision eventually resulting in complete night blindness witnessed in patients suffering from retinitis pigmentosa has led to the conclusion that the rods are degenerating more rapidly and to a greater degree than the cones. Hence, some vision is retained in serious cases of retinitis pigmentosa thereby indicating that some cone function continues even after degeneration of the rods. It is only in the very last stages of retinal degeneration that the surviving cones begin to deteriorate.
Consequently, in addition to preventing the eye from being exposed to bright illumination and substantial changes in light levels, the rods should be more protected than the cones. The photopigment concentration of the rods should be maintained at relatively high values to inhibit the production of bleaching products caused by exposure to light. Therefore, in order to reduce the deposition of worn out cells of the laminar layers of the outer receptor segments which normally are continuously absorbed, protective lenses should transmit light intensities in only such wavelengths that are primarily outside of the absorption range of the rod pigment. Hence, if a spectacle lens could be developed which would permit sufficient light to enter the eye for acceptable vision via the cones alone, it might significantly decrease or even inhibit further retinal degeneration.
Those requirements for reducing retinal degeneration occurring in retinitis pigmentosa could be answered via the use of pure red spectacle lenses. Nevertheless, such lenses would be unsatisfactory from a practical point of view because their monochromaticity does not permit even a moderate degree of natural color vision, e.g., distinction between red and green signal lights, inasmuch as the lenses excessively narrow the spectral information presented to the photoreceptor by the retinal image. Therefore, to impart sufficiently good color fidelity to the lenses, some transmittance in the green portion of the spectrum must be present.
Finally, it would be very beneficial to have the spectacle lenses designed for aphakics and for those persons subject to photoretinitis or retinitis pigmentosa prepared from glasses demonstrating photochromic behavior. Such glasses would act to compensate for differences in indoor and outdoor radiation intensity levels. If desired, polarizing capability can also be incorporated into the spectacle lenses as, for example, by attaching a film of polarizing material to the rear face of the lens. Such action would combine the well-recognized benefits of polarizing eyewear with the advantages of the short wavelength filter characteristics of the present inventive glasses.